ELECTRIC PROPULSION OF SHIPS. 139 



when the magnetic air-gap clutch takes care perfectly of all fractional speeds even down to 

 complete stopping of the tail shaft? 



The provocation for use of electric propulsion existing with turbines which will not re- 

 verse and are involved in such destructive superheating troubles whenever they are forced to 

 go through this maneuver, does not exist with Diesels. The Diesel will run in either direc- 

 tion equally well and responds perfectly and instantly to the reversing maneuver. 



As we all know, the reciprocating steam engine, barring foaming boilers and condensation 

 troubles, makes by far the best engine for ship propulsion. It maneuvers perfectly and, while 

 it has complete flexibility, yet it will give instantly full power astern or ahead at will. Com- 

 pared with this performance, for instance, just what is supposed to be the contribution of 

 Diesel-electric propulsion? 



1. It is supposed to give both the naval architect and engine builder complete freedom 

 in design to employ the best speed for propeller and engine. This would be realized were it 

 not for the fact that both the electric generators and motors are too heavy and expensive 

 if these speeds are kept low, so there is a constant urge toward too high speeds and the actual 

 plants are a compromise at best. This is not at all true with gears, where the lower speeds 

 are encouraged. 



2. It gives divided prime mover units. Should an emergency arise, a part only of the 

 plant is crippled. This is true with the gears and the air-gap clutches at a very great saving of 

 weight, expense and space. Here a plurality of engines is always present. (See Fig. 5, 

 Plate 27.) 



3. Flexibility. — The Diesel will rim with as great reliability as steam at all speeds but 

 the lowest. Here the Diesels are found to draw too heavily on the starting air reserves, as 

 stated, so they may be left running at full or fractional speeds while the electric transmission 

 does the rest and allows the propeller shaft full range down to just turning over if necessary 

 for slow headway. This is precisely the function of the loaded secondary in the air-gap 

 clutch. The continuous operation at all fractional speeds is provided for and at many times 

 the efficiency of the electric drive and without its weight and expense; and as to the impor- 

 tant matter of space, with a plant located within the flywheel of the engine itself. 



4. It is supposed to make the farther doubtful contribution of avoiding reversing oil 

 engines. Reversing Diesels were formerly so complex that there was little wonder that re- 

 versing relief was sought in the electric plant, but this has now been done away with. For 

 instance, the 32 cams for each four cylinders for full air starting and reverse has in the new 

 engines been reduced to 5 and the problem no longer exists.* The maneuverability with these 

 engines and the air-gap clutches accomplishes all that the full electric plant does, at a very 

 great saving of plant and control equipment. 



Taking up the detailed construction of these clutches, a brief statement might be made 

 as follows : 



The clutch is characterized by two kinds of torque operating by opposite phenomena ; 

 the greater the differential or relative velocity between the driver and driven parts (so indi- 

 cated in Figs. 2 and 3, Plate 27), the greater the torque available for starting and for bring- 

 ing up to synchronism. This phenomenon also provides for slipping and continuous operation 

 at all fractional speeds by means of the loaded secondary effect acting as an induction motor. 

 The electrical conductor in the loaded secondary is indicated by the large light masses in the 

 driven rings separating the blackened masses, which indicate the magnetic inserts. (Fig. 3.) 



The air-gap clutch drives by means of magnetic flux. There are four elements in the 



*See author's A. S. M. E. paper, "Compound Combustion Engines," Annual Meeting, 1921. 



